The invention is particularly, but not exclusively, applicable to the field of valve devices designed to feed a flow of gas which can be regulated in a particularly accurate and precise way, where there is a requirement to provide rather extensive modulation of the range of feed flow rates. An example of a typical application of this type is a heat and power generating system based on the use of fuel cells. In these systems, heat is generated by processes of combustion of combustible mixtures, and the gas is also converted directly to electrical energy by electrochemical processes in the fuel cells. In these processes, the flow rate of the combustible gas (such as natural or liquid gas) has to be regulated accurately, especially at the minimum flow rates required in the desired modulation range. Known solutions provide for the use of gas feed lines of the “open circuit” type, in which the feed flow rate is directly correlated with the inlet pressure of the gas from the gas supply mains. However, this gas inlet pressure is difficult to keep under control, since it is subject to fluctuations caused by conditions in the mains. Consequently, the supply system along the line becomes unstable and inaccurate, especially at pressures close to the minimum feed pressure (and flow rate).
Moreover, the outlet pressure can also be subject to fluctuations which are difficult to control. For example, if the outlet cross section is made to communicate with the restricted section of a Venturi tube, provided to mix the gas fed by the valve with air or other air-like substances, the feed pressure of the valve device may be subject to fluctuations which can have a significant effect on the feed flow rate.